The present invention relates to a flat or tubular film with a base layer which may be fiber-reinforced and is based on cellulose hydrate, preferably to a packaging film, in particular to a tubular artificial sausage casing, and to a process for manufacturing the same.
Flat, that is to say web-shaped or sheet-like, as well as tubular films, based on cellulose hydrate have been known for a long time and are usually produced by the viscose process. An alkaline solution of cellulose xanthate, generally termed viscose solution, is extruded through an annular die or slot die and coagulated and regenerated to give cellulose hydrate by means of an acidic fluid. The properties of the tubular or flat body of regenerated cellulose can be varied by the composition of the viscose and incorporation of additives. The unreinforced cellulose hydrate films are also called transparent cellulose films and are known under the trademark Cellophane. In the production of fiber-reinforced bodies, a tubular or web-shaped fiber material is coated and impregnated with viscose solution on one or both surfaces and then treated in a conventional manner with coagulation fluid and regeneration fluid. It is also known to bend web-shaped cellulose films, after they have been produced, to form tubing and to join the overlapping edges to one another, forming a longitudinal seam.
Films of this material show good water absorption capacity and are also permeable to water vapor. They are therefore widely used as artificial sausage casings for dry sausage such as, for example, long-life sausage or salami, and used as packaging film.
It is known that cellulose hydrate films become increasingly brittle and hard during storage due to loss of water and plasticizer. This phenomenon is to be ascribed to crystallization occurring in the cellulose material, which leads to the formation of hydrogen bonds in the cellulose structure between the individual cellulose molecules and hence to closer proximity and spatial fixation of the molecules. The consequence of this process of crystallization and structural change is a deterioration, progressing in time, of certain physical properties, in particular of the expansion, strength and swelling value of the film. In addition, the crystallization process causes pronounced shrinkage of the film so that, in the case of tubular packaging based on cellulose, a great increase in the pressure exerted on the enveloped product is to be found during storage.
This process of structural change, gradually proceeding in cellulose hydrate films, and the associated disadvantageous brittleness of the film can be prevented only to a limited extent by the addition of water-soluble, so-called secondary plasticizers. Secondary plasticizers such as, for example, glycerol, propylene glycol or polyglycol, can admittedly improve the softness and handle of the shaped body. Since secondary plasticizing agents are not bound by a chemical bond to the cellulose hydrate molecules, but only by intermolecular forces, they tend to migrate out of the film or are dissolved out of the casing material, especially when the film is put into water, such as is usual for sausage casings made of this material before they are filled with sausage meat, and on scalding and cooking of the sausage.
The consequence is a particularly pronounced brittleness of the plasticizer-free dried cellulose material. In the case of sausage casings with a water vapor barrier layer on the inner surface, the moisture from the sausage meat cannot pass into the cellulose layer. As a result, such casings become shock-sensitive and tend to tear over the entire length when the sausage is cut for the first time. It is therefore typical to moisten briefly sausages having such a sausage casing with cold water before they are cut for the first time. However, this measure cannot prevent, during storage of the cut sausage, bulging of the sausage meat out of the cut surface due to the high internal pressure generated by the dry casing or even it being forced out of the sausage casing at the cut surface.
A further disadvantage of cellulose-based sausage casings manifests itself in the maturing of dry sausage, for example salami. Uniform maturing of these sausage-types demands a very slow release of water from the sausage meat during the first days of maturing. When conventional fiber-reinforced cellulose casings are used, a very high relative atmospheric humidity must prevail in the surroundings for optimum maturing of the sausage, and fluctuations in atmospheric humidity must also be avoided. For this reason, the maturing of dry sausage having a conventional sausage casing of cellulose hitherto had to take place in maturing chambers, in which the relative atmospheric humidity had to be carefully controlled within comparatively narrow limits.
An unduly low or fluctuating moisture content of the surrounding air during maturing of the sausage leads to so-called dry edges on the periphery of the sausage. This undesired phenomenon occurs particularly when the sausage meat dries out too quickly at the outer surface of the sausage in the first days of maturing. The dried outer periphery of the sausage, the so-called dry edge, prevents the further escape of moisture from the core of the sausage, so that the sausage meat in the interior of the sausage is still moist after the usual maturing time and lags behind in maturing. Moreover, the sausage casing no longer adheres sufficiently to the dry edge, so that undesired interspaces between the sausage meat and the casing and creases in the casing are formed.